Field of the Invention
The present invention relates to a new and improved electrically conductive resinous bond, its method of manufacture, and its use in the high temperature fabrication of electrical devices, e.g., the bonding of crystal blanks to mounting clips in crystal resonator fabrication, integrated circuit die bonding, tuning fork fabrication in electronic watches and the like. More specifically, the present invention is directed to a bonded article and method of manufacture utilizing an unfilled polyimide resin in combination with a conductive metal-filled polyimide resin, for providing a bond between elements, which is electrically conductive and exhibits a high resistance to shock and vibration. Advantageously, the bond of the present invention must operate effectively in high temperature and hard vacuum (10.sup.-6 to 10.sup.-8) torr applications and must provide a bonded, void free, dense, resilient joint which, because of its resiliency, functions with less strain, and in the case of crystal resonators, provides a better long term crystal frequency stability.
A presently known technique for attaching a quartz crystal blank to a mounting structure (clips) employs the use of a silver-filled polyimide resin. The conductivity of the bond, which is essential to the operation of the crystal, is provided by the silver present in the bond. However, recent needs have created a demand for a conductive bond composition which possesses high shock and vibration resistance which exceeds the yield point of the bond strength of the known prior art materials by a considerable margin. The silver-filled polyimide resins referred to above do not possess the necessary resistance to vibration and shock. Gold-filled compositions have been investigated in this regard, but it has been found that they provide only about double the strength of the silver-filled polyimide resin and also have a low process reliability. Since gold is about twice the weight of silver, the gold-filled polyimide resin contains approximately 33% more polyimide than the silver-filled polyimide resin and this factor accounts for the increased strength.
U.S. Pat. No. 4,233,103 to Shaheen is directed to a high-temperature resistant, conductive adhesive composition for bonding a semiconductor die to a substrate which utilizes a mixture of a eutectic alloy of gallium-tin and gold and a polyimide resin. However, the metal matrix subsequently formed is structurally rigid and brittle, and when used in quartz resonators, could result in adverse effects with extremes of shock and temperature.
U.S. Pat. No. 4,284,461 to St. Clair et al discloses a method for preparing an aluminum ion-filled polyimide adhesive in which a meta-oriented aromatic diamine is reacted with an aromatic dianhydride and an aluminum compound in the presence of water or a lower alkanol miscible ether solvent to produce an intermediate polyamic acid. Thereafter, the polyamic acid is converted to the thermally stable, metal ion-filled polyimide by heating, and the result is a flexible, high-temperature adhesive. This adhesive, however, does not qualify as an electrically conductive adhesive when used alone because of the low metal content.